Dispositional optimism shares a robust link with better physical and mental health. Optimistic people do many things right?they exercise, they go to the doctor, they maintain healthy diets, and they avoid deleterious health behavior (e.g., smoking, drinking). Recent research has shown that the optimism of people in one's immediate social network can improve the health of an individual. Having an optimistic partner predicts better health, fewer chronic illnesses, and fewer physical limitations among older adults. Nevertheless, the mechanisms for why other people's psychological characteristics affect an individual's health and well-being are unknown. Does having an optimistic partner encourage the adoption of preventative health behavior? Can the benefits of having an optimistic partner be seen on the biological level and predict mortality? The current proposal investigates the longitudinal effects of actor and partner optimism on changes in health and biological functioning over a six- and eight-year interval by employing a dyadic mediation model in two cohorts of nearly 2,000 couples each. The project is innovative in that it will use state-of-the-art methods for modeling dyadic data and examining the mediating processes linking optimism to better health and biological functioning. The unique design of the HRS provides longitudinal data on health, biomarkers, and mortality from a large number of older couples. Identifying and testing the mechanisms that link optimism with better health and biological functioning in older couples will help clinicians develop individual and couple-based interventions to enhance health in late life.